The present invention relates to generally an echo sounder for detecting schools of fish, and more particularly a circuit thereof for processing the signal to be applied to an electrical discharge type recorder.
In general, even when the level of the signal to be applied to a discharge type recorder is linearly changed, the density of the image recorded on a recording paper will not change linearly. Therefore when the electrical discharge type recorder is incorporated in an echo sounder for detecting schools of fish, the images of schools of fish near the surface of the sea or the sea bottom cannot be distinctly discriminated from the surface of the sea or the sea bottom which is displayed as the dense image or from the air bubbles near the surface of the sea or from the rocks at the sea bottom.